This invention relates to an electrical filter, and more particularly, it relates to a surface acoustic wave (SAW) filter wherein an intermediate structure between input and output transducers reduces cross-coupling and/or interference.
An acoustic wave filter, used as a resonant filter, which employs input and output transducers arranged directly adjacent to each other on the surface of the substrate has been described in the Proceedings of IEEE, Volume 67 (1969), Page 147-157, FIG. 3. Even with this described polarization arrangement, however, a disturbing degree of cross-coupling between input and output is present. Naturally, such cross-coupling reduces the effective operational characteristics of the SAW filter by providing an unwanted shunt signal path.
In order to provide shielding between the input and output transducers, various arrangements have therefore been provided, as are described in "1982 Ultrasonics" Symposium 1982 IEEE, Pages 284-289, FIG. 6. Basically therefore, such a shielding arrangement makes use of a ridge-like structure, oriented transverse to the principal propagation axis of the waves travelling in the filter which is provided as a surface coating for the substrate and that also serves as a ground potential reference. This surface coating, which is at ground potential, forms a broad stripe positioned between the input and output transducers and has good shielding properties. At relatively high frequencies, however, the effect of the inductances of the leads on this shielding becomes significant as a disadvantage and stray potentials develop on the coated shielding which cause interference between input and output terminals of this filter.
Additional reference is also made to Ultrasonics Symposium 1979 IEEE, Pages 841-844.